In a froth flotation process, the material to be processed is present in a suspension in water. A reagent is added which renders the particles to be separated hydrophobic or non-wetting with water. This reagent is known as a “collector”. Air bubbles are introduced into the suspension in a flotation cell or column, and on being brought into contact with the particles by collision, attach to the hydrophobic particles and carry them to the surface of the liquid where they form a froth. The froth layer flows out of the flotation cell into a froth overflow launder. The particles which have not adhered to bubbles flow out of the cell in a tailings stream. The unwanted particles are typically referred to as the “gangue” (in minerals) or “ash” (in coal processing) particles.
The flotation process is typically applied in the coal and minerals industries, to particles less than 300 to 500 micrometers in diameter. In some cases, it would be advantageous to be able to recover particles larger in size, especially in coal processing, where particles in the range 300 to 2000 micrometers are typically separated using other technologies, which exploit the difference in density between the coal and the ash material. Flotation has not generally been applied to the flotation of particles above 300 to 500 micrometers, because the efficiency of recovery of valuable particles above this size is typically very low. The reason is that in normal flotation, the particles to be separated and recovered must first become attached to air bubbles which lift them out of the slurry in the flotation cell and into the froth layer above the slurry.
It will be appreciated that in order to rise, the density of the bubble-particle aggregate must be less than that of the surrounding slurry, and the larger the particles, the larger must be the total volume of gas bubbles attached to the particles to achieve buoyancy. In practice, it is observed that aggregates which form between large particles and single bubbles or clusters of bubbles are easily dispersed by the fluid mechanical forces due to turbulence in the slurry in the flotation cell. Accordingly, there is a high probability that particles larger than 300 to 500 micrometers in diameter will remain in the slurry and pass out of the cell with the gangue or ash material in the tailings, even though they may be truly hydrophobic.
One way of improving the flotation of coarse particles is to introduce them into the froth layer on top of the flotation cell. If such particles, which have already been made hydrophobic by suitable treatment with a collector, can make contact with the bubbles in the froth, there is a high probability that they will remain in the froth and be recovered into the froth overflow launder, whereas coarse particles of gangue or ash material will pass through the froth into the slurry below, to be discharged with the tailings.
A difficulty with the attempts that have been made in the past to float coarse particles in the froth, is that the slurry incorporating the coarse particles was introduced on to the upper surface of the froth from a central distribution point. However, this results in a very uneven distribution of coarse particles in the froth and has not been found to be effective.
One of the problems inherent in the froth flotation process is the entrainment of unwanted matter by the bubbles rising into the froth layer. These particles report to the froth concentrate leaving the cell, and cause a reduction in the quality or grade of the flotation product. In general, the amount of entrainment in the froth concentrate is proportional to the volume of water recovered in the froth. One way of reducing or eliminating the amount of entrained material is to apply wash water to the top of the froth. The wash water drains downward in the froth layer, and flushes the unwanted particles back into the flotation cell, whereas the hydrophobic particles, being attached to the bubbles, are able to flow upwards and out of the cell.
The means for the distribution of wash water in flotation cells typically consists of a shallow tray drilled with small holes at regular intervals, and placed a short distance above the froth layer. Water is fed to the tray, and passes through the holes to form a multiplicity of jets or droplet streams which fall on top of the froth. Variations include systems of perforated pipes which can be placed above or within the froth layer. Water is introduced into the pipes, and flows or drips out of the perforations in the pipe wall, into or above the froth. It will be understood that any apparatus involving the passage of the wash water through small holes or orifices will be prone to blocking by adventitious particles which may be present in the wash water, giving rise to a reduction in the efficiency of distribution of the wash water, and requiring frequent maintenance and inspections to prevent or remove blockages.
In practice, it can be difficult to obtain clean process water in coal washeries and mineral concentrator plants for use as wash water. Often, the process water is obtained as the overflow from thickeners or settling ponds, and if there is a malfunction in the plant which interrupts the water clarifying process, the water which is recirculated back into the plant can contain significant quantities of fine particles, sometimes as much as five to ten percent by weight. These particles can settle readily in regions of low velocity in the wash water delivery pipes, or in wash water trays, and block the holes or perforations intended to allow the water to be distributed in the froth. It would be advantageous to be able to supply a process and apparatus for distribution of wash water which was not prone to blockage by small particles and which is capable of operating for long periods without failure due to blockage of holes.